One problem which the invention solves is understood with respect to one close tolerance manufacturing application: The grinding to length of the tips of gas turbine engine blades mounted in a spinning rotor. The process is generally described in Miller U.S. Pat. No. 4,512,115. The blade tips pass very quickly by both the grinding wheel contact point and the measurement point; typical times of passing are of the order of 0.004 seconds. Thus, a very quick and accurate means of measuring is required.
As described in Drinkuth et al. U.S. Pat. No. 4,501,095, a method and apparatus for accomplishing the necessary measurement comprises a laser triangulation system. Laser light is read and measurements are made for each individual blade, which measurements were not possible previously, given the high speeds at which blades pass by. A control system guides the grinding apparatus accordingly, and a uniform and precise degree of metal removal is achieved.
In process of the Drinkuth et al. patent, the light intensity impinged on the blades is controlled by a motor driven cam type mechanical aperture that blocks part of laser light emitted from the source. The intensity control seeks to make the refelected light neither too high nor too low for the position sensor. If the reflected light intensity and resultant sensor signal level is too low, the signal to noise ratio will be low and there will be resultant measurement error. On the other hand, if the reflected light intensity is too large, then the sensor will become saturated, resulting in imprecise computation of signal centroid with concomitant measurement error.
The response rate of familiar mechanical aperture devices is low, at the order of 1 Hz, compared to the 250 blade/s and greater rate at which blade tips pass by the measuring point. Thus, heretofore, the aperture has been necessarily set to provide some average light intensity for blade grinding. Such system worked satisfactorily on new rotor systems.
However, with evolving blade technology and with re-manufacturing, it has been discovered that blade tips may now vary substantially from one to the next in their surface reflectance character, compared to the general uniformity which characterized the original context of manufacturing new products. For example, some blade tips are coated; and, for a mixture of old and new blades, some will be dull and others shiny.
When using an averaged laser light intensity, for such mixtures of blades there will be erroneous measurements, defeating important objects of the original inventions. When brighter blades influence the average intensity setting, very dull blades may not be sensed; and, an absence of signal can lead to inoperability of the system, or the necessity for guessing. Analagous imprecision will occur for bright blades when the average intensity is influenced by very dull blades, for reasons given above. A not unusual maximum-minimum variation in reflected intensity is of the order of 50:1, when using an average or comprise level of intensity. And, with such a limitation, the invention is limited in being applied for other high speed measuring applications.
Thus, there has been a need for a better means of measuring blades in the prior art grinding method; and, in other measuring systems which require the essential aims of rapid accurate measurements on parts which are only momentarily available for gaging.